Simon Pegg (Shaun of the Dead, Star Trek) has signed on to reprise his role as the comic relief lab geek Benji Dunn for Mission: Impossible 4. THR reports that while the project has yet to be officially greenlit, director Brad Bird has been reading actors this week and production is tracking to begin in early September 2010.

Brad Bird, the two-time Oscar-winning filmmaker behind Pixar’s The Incredibles and Ratatouille, and the critically acclaimed 2D animated feature The Iron Giant, won out over a reported list of other contenders which included Zombieland director Ruben Fleischer and Shaun of the Dead/Hott Fuzz helmer Edgar Wright to direct.

When the project was first announced, there was a lot of speculation that it would be a reboot, with a new star in the front and center spy role. But when Paramount Pictures signed Tom Cruise to reprise his role as Ethan Hunt in M:I-IV, it was revealed that the film would be another sequel — with Cruise’s character as the lead. It was revealed that Tom and JJ developed “a really cool idea” for a fourth adventure, which Abrams turned into “an insanely long outline”.

Alias‘ scribes Josh Applebaum and Andre Nemec recently turned in their script and the studio executives seemed to really respond to it. The disappointing box office take of Cruise’s Knight and Day has resulted in rumors that the script might be reworked to allow for another actor to play a young operative for Hunt would mentor, although studio sources have denied this.

Mission Impossible 4 is being developed for an announced December 16th 2011 release.